The present disclosure relates to subject matter contained in priority Japanese Patent Application No. 2002-121571, filed on Apr. 24, 2002, the contents of which is herein expressly incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a sealed rechargeable battery. More particularly, it relates to a battery having an electrode plate group constructed with a positive electrode plate and a negative electrode plate wound around with a separator interposed therebetween and accommodated in a case having an oval or rectangular cross section.
2. Description of Related Art
Demands have been growing for electric vehicles or gasoline-electric hybrid vehicles in light of global environmental issues. Rechargeable batteries used as power sources for such vehicles are required to be small and lightweight as well as to have high capacity and high power output. Lithium ion rechargeable batteries and nickel metal hydride batteries are known to satisfy these requirements and have actually found practical applications.
FIG. 6 shows one such conventional sealed rechargeable battery. It accommodates an electrode plate group 24 in a cylindrical metal case 25 together with liquid electrolyte. The electrode plate group 24 consists of a strip of positive electrode plate 21 and a strip of negative electrode plate 22 wound around with a separator 23 interposed therebetween. Positive and negative electrode tabs 26 and 27 are extended from given locations of the positive electrode plate 21 and negative electrode plate 22 in opposite directions. The negative electrode tab 27 extended to the lower side is welded to a bottom face of the case 25, and the positive electrode tab 26 extended to the upper side is welded to a sealing plate 28 for sealing the open end of the case 25. The sealing plate 28 is fixed to the case 25 with a gasket 29 fitted therebetween to provide a seal.
In the construction shown in FIG. 6, because the positive electrode plate 21 is connected to the sealing plate 28 via the positive electrode tab 26 and the negative electrode plate 22 is connected to the case 25 via the negative electrode tab 27, the connection resistance is large. The battery internal resistance is accordingly large, because of which the output characteristics and large current discharge characteristics are not satisfactory. There is also the problem of high costs because of large numbers of components and assembling steps.
Japanese Patent Laid-Open Publication No. 2000-243433 discloses a construction for a rechargeable battery, which is shown in FIG. 7A. A positive electrode plate 31 and a negative electrode plate 32 are wound around with a separator 33 interposed therebetween to form an electrode plate group 34 such that one lateral edge of the positive electrode plate 31 and one lateral edge of the negative electrode plate 32 are respectively protruded. These projecting portions 31a and 32a of the positive and negative electrode plates are directly bonded to a sealing plate 36 and the bottom face of a case 35, respectively, by laser beam welding. To achieve high bonding strength in the welding between the positive electrode projecting portion 31a and the sealing plate 36 and between the negative electrode projecting portion 32a and the bottom face of the case 35, radial ribs 37 are provided on the sealing plate 36 or on the bottom face of the case 35 so as to form protruding bonding surfaces as shown in FIGS. 7B and 7C.
The battery shown in FIGS. 7A to 7C, however, has the following problems: When constructing a battery pack by connecting a plurality of such batteries as battery modules, the number of the batteries that can be accommodated in a confined space is limited because they are cylindrical. The volume efficiency will be further reduced if cooling passages are to be formed between the batteries to prevent battery temperature rise. Each battery will exhibit poor cooling properties particularly in the central portion because its cross section is circular. That is, this battery has poor volume efficiency per power output.
Another problem is that, because the electrode plate group 34 is welded to the case 35 and the sealing plate 36 at the radial ribs 37 as shown in FIG. 7B and FIG. 7C, welds are gathered in high density in the central portion of the electrode plate group 34 while the welds are sparse in the outer peripheral portion. This makes the current distribution in the positive and negative electrode plates 31 and 32 uneven, which increases battery internal resistance and makes the chemical reactions of active materials uneven. The output characteristics and large current discharge characteristics are thereby reduced, adversely affecting the battery life.
Japanese Patents Nos. 2692533 and 2697565 show a construction for a sealed prismatic rechargeable battery, in which an electrode plate group formed by winding around a positive electrode plate and a negative electrode plate with a separator therebetween is accommodated in a prismatic case such that its winding axis is in parallel to the open end of the case. The open end of the case is sealed by a lid provided with a positive terminal, to which one end of the positive electrode plate is connected. The negative electrode plate which is located at the outermost side of the electrode plate group is connected to the case. With this construction, because one end of the positive and negative electrode plates is connected to the positive terminal and the case which forms the opposite terminal, the current path from each electrode plate is relatively long, so that the battery internal resistance is large because of the large current resistance inside the battery, and therefore the output characteristics and large current discharge characteristics are not satisfactory. Moreover, while a cylindrical battery can retain its electrode plate group securely in position within the case, the prismatic battery cannot hold its rolled electrode plate group securely inside the rectangular tube case. Thus, there is a risk that active material mixtures may come off the positive or negative electrode plates when vibration is applied to the battery, whereupon the battery characteristics are suddenly deteriorated because of the damaged electrode plate group.
In light of the problems of the conventional batteries, an object of the present invention is to provide a sealed rechargeable battery having high volume efficiency per power output and favorable output characteristics and large current discharge characteristics.
The present invention provides a sealed rechargeable battery including: a metal case having any of an oval cross section and a rectangular cross section; a metal sealing plate for sealing an open end of the case; an insulation gasket interposed between the case and the sealing plate; and an electrode plate group including a positive electrode plate and a negative electrode plate wound around with a separator interposed therebetween. Core materials of the positive electrode plate and the negative electrode plate are protruded to opposite sides to form a positive electrode end face and a negative electrode end face at either end of the electrode plate group. The electrode plate group is accommodated in the case with liquid electrolyte with its winding axis vertical relative to the open end of the case. The positive electrode end face is connected to either one of a bottom face of the case and the sealing plate in direct surface contact therewith or indirectly coupled thereto, and the negative electrode end face is connected to the other one of the bottom face of the case and the sealing plate in direct surface contact therewith or indirectly coupled thereto.
With this construction, because the battery case has an oval or rectangular cross section, its volume efficiency per power output is higher than that of a cylindrical battery. A plurality of such batteries can be arranged in a limited space with good space efficiency particularly when constituting a battery pack with cooling passages between adjacent batteries. The resultant battery pack will thus have a small volume per power output. The battery exhibits high cooling properties relative to the entire cross section because of the flat outer shape, and its metal case has better cooling properties than a resin case, whereby temperature rise of the battery is effectively prevented and the battery output characteristics and service life are improved. Furthermore, because both end faces of the electrode plate group are respectively connected to the bottom face of the case and the sealing plate in a state wherein they make direct surface contact with each other or indirect connection therebetween, the current path between any given point in the electrode plates and the case or sealing plate which will form external connection terminals is made short, and also the current distribution in each electrode plate is made uniform, whereby the active materials on the electrode plates are entirely and uniformly contributed to the reactions. Internal resistance is thus reduced, high output characteristics and large current discharge characteristics are achieved, and battery life is made longer. Also, since both end faces of the electrode plate group are held entirely by the bottom face of the case and sealing plate, the electrode plate group is securely retained even when vibration is applied. Therefore there is no risk that battery characteristics are suddenly deteriorated resulting from damages to the electrode plate group because of active material mixtures coming off the electrode plates. Moreover, because the metal case does not necessitate any treatment for preventing gas permeation as with a resin case, the battery is fabricated with fewer process steps and at lower cost.